Here's the theorycraft behind Gravity versus Lightspeed + Gravity.

Gravity + Lightspeed

Gravity Normally
@00.00s - Begins to cast first Gravity
@03.00s - First Gravity resolves, begin to cast second (total potency - 200 X multi-enemy multiplier)
@06.00s - Second Gravity resolves, begin to cast third (total potency - 400 X multi-enemy multiplier)
@09.00s - Third Gravity resolves, begin to cast fourth (total potency - 600 X multi-enemy multiplier)
@12.00s - Fourth Gravity resolves, begin to cast fifth (total potency - 800 X multi-enemy multiplier)

Gravity w/ Lightspeed
@00.00s - Cast first Gravity (total potency - 150 X multi-enemy multiplier)
@02.50s - Cast second Gravity (total potency - 300 X multi-enemy multiplier)
@05.00s - Cast third Gravity (total potency - 450 X multi-enemy multiplier)
@07.50s - Cast fourth Gravity (total potency - 600 X multi-enemy multiplier)
@10.00s - Cast fifth Gravity (total potency - 750 X multi-enemy multiplier) [Assuming you get the Lightspeed in for easy math]
@12.50s - Begin casting sixth Gravity

Gravity Normally is 800 potency at the 12s mark for a total 66.67 potency per second (plus multipliers which is the same across the board)
Gravity w/ Lightspeed is 750 potency at 10s mark for a total of 75 potency per second (plus multipliers)
Gravity w/ Lightspeed is 750 potency at 12s mark for a total of 62.5 potency per second (plus multipliers)

In theory this looks better to do Gravity + Lightspeed when you consider number of casts within the 10s period of Lightspeed but since you can't begin to cast a sixth Gravity until the 12.5s mark and won't get it until the 15.5s mark, you might as well count Gravity w/ Lightspeed as 5 Gravity's over 12s, leading to the above. Therefore it's better not to use Lightspeed with Gravity.
750/800 = 93.75% aka a 6.25% DPS loss. It doesn't matter the number of targets you target since the same % reduction would apply to every additional mob past the first you'd hit.

If you consider Gravity as a DPS tool in the exact same scenarios where you may use Gravity + Lightspeed versus just Gravity by itself, Gravity will win in out right damage (and you'd still end up using the same MP anyway since you need five Gravity's instead of four just to keep up in damage).

Considering the context of the posts above indicated that using Lightspeed Gravity is a greater DPS increase versus just using Gravity by itself, I'm just stating that this particular idea is wrong (as per above quote)

If you're looking at a myriad of other scenarios listed above, I'd say it's up to the individual healer to understand their kit and use the appropiate toolkit for it and have no bearing to the actual context of "Lightspeed + Gravity DPS" > "Gravity DPS"

If you wish to front load a heal due to lightspeed that's your decision, but it's not how I would use Lightspeed in most trash-based scenarios.

Take into account that unlike Presence of Mind, Lightspeed doesn't reduce your recast so the only DPS spell that would gain any benefit from the cast time reduction would be Gravity. Any healer spell would be front loaded but you're still restricted to the same 2.5s recast.